This invention relates to curable elastomer-modified epoxy resin compositions and to coatings made from such compositions. In a specific aspect, the invention relates to a method for dispersing an elastomeric block copolymer in an epoxy resin coating formulation.
Rubber-modified epoxy resins are well known. Rubber modification, by blending or co-reacting epoxy resins with elastomeric compounds, is employed to improve such physical properties of epoxy resins as toughness, low profile characteristics, flexibility and adhesion. Rubber-modified epoxy resins are used in formulating corrosion and chip-resistant coatings and impact resistant molding powders.
Powder coatings are used in the automotive industry as primer coatings. Automobile and truck bodies, particularly wheel wells and lower portions of the bodies such as rocker panels, are susceptible to chipping by sand and gravel.
Numerous organic solvent-based coating compositions useful as chip-resistant primers have been proposed as, for example, in U.S. Pat. Nos. 4,581,424, 4,602,053, 4,608,313, and 4,614,683. However, these coating compositions have a high volatile organic compound (VOC) content, which poses a problem for industries attempting to meet governmental guidelines on VOCs.
Chip-resistant powder coatings, which have the advantage of very low VOC, containing carboxy-terminated butadiene acrylonitrile copolymers (CTBN) are known. But, it has been found that the CTBN modifier in the epoxy resin contributes to stability problems and difficulties in processing due in part to its low T.sub.g and its chemical instability.
Carboxy-modified butadiene-styrene block copolymers ("block copolymers") are known to improve the mechanical strength of an epoxy resin composition. Japanese patent applications 58-17160 and 63-308027 disclose that carboxy-modified butadiene-styrene block copolymers give good impact resistance and rigidity in a heat-curable molding composition. However, it has been found that some of the epoxy resins modified with carboxy-modified butadiene-styrene block copolymers phase separate and precipitate upon curing or storage at ambient temperature. In order to obtain a toughened resin composition, the two components need to be incompatible enough to form a dispersed domain of fine rubber particles in the epoxy resin matrix. At the same time, the rubber particles must not precipitate upon storage or curing. CTBN copolymers give some undesirable properties, e.g. stickiness and chemical instability, to the epoxy resins but are relatively compatible and soluble in the resin. Carboxy-modified butadiene-styrene block copolymers are normally incompatible in epoxy resin and readily form precipitates which renders them difficult to handle in a form useful in epoxy coating compositions, as the precipitates tend to form clumps and give an undesirable uneveness to the coated film. It would be desirable to achieve the advantages of the block copolymer modification of an epoxy resin without the dispersion and precipitation disadvantages of such modification.
It is therefore an object of the invention to provide an elastomer-modified epoxy resin composition useful in coating applications. It is a specific object of the invention to provide a block copolymer-modified epoxy resin composition which is stable and readily processible in coating applications.